Flash A/D converters, semi-flash A/D converters, parallel-sequence A/D converters, and some other types of A/D converters contain large numbers of matched comparators, which number is fixed by the number of bits in the system and the structure of the system. In the case of an 8-bit flash A/D converter, the number of matched comparators is 256.
All of the comparators in the converter must be tested to guarantee the performance of the converter. Normally, A/D converters can be tested by several methods, but all of these methods require multiple test steps for each comparator in the converter. If a resolution of 0.05 for a least significant bit (LSB) is required for the converter, the test steps for each comparator becomes 20. Thus, the total number of steps required to fully test the converter are 5120. If, for example, one millisecond is needed for one test step, the total time required to complete the test of the converter is 5.1 seconds. This test time is very long in mass production. Reducing this test time is a key to production cost reduction.